In a known inverter device construction, as shown in FIG. 1, free-wheel diodes Du to Dw and Dx to Dz are connected in anti-parallel with MOSFETs Su to Sw and Sx to Sz. In this construction, when the MOSFETs Su to Sw and Sx to Sz are turned OFF, the current energy accumulated in the load M is fed back through the free-wheel diodes Du to Dw and Dx to Dz.
In this case, when for example the MOSFET Su is turned ON when a forward current Ia flows in the free-wheel diode Dx, the PN voltage (so-called DC link voltage) is applied as reverse bias to both terminals of the free-wheel diode Dx, with the result that, as shown in FIG. 2, a reverse-direction current due to the residual charge flows in the free-wheel diode Dx before the free-wheel diode Dx cuts off. Considerable loss was therefore generated in the free-wheel diode Dx by the PN voltage and reverse-direction current, so it was necessary to employ a large heat radiator.
Measures were therefore taken to reduce the loss generated in the free-wheel diode by providing a reverse voltage application circuit, so that when the free-wheel diode was cut off, a small reverse voltage could be applied from this reverse voltage application circuit to the free-wheel diode, so that reverse recovery of the free-wheel diode could be brought about by the low-voltage DC voltage source of this reverse voltage application circuit. Such an invention is described in for example Laid-open Japanese Patent Application No. H. 10-327585, which is a Japanese Laid-open Patent Application.
FIG. 3 is a circuit diagram of a prior art power conversion device comprising a reverse voltage application circuit. In FIG. 3, the DC voltage source 1 is obtained by rectifying a three-phase AC power source; a smoothing capacitor 2 and a main inverter circuit 3 are connected between the positive side DC bus 1a and the load side DC bus 1b of this DC voltage source 1. The main inverter circuit 3 comprises a three-phase bridge connection of MOSFETs 4u to 4w, 4x to 4z corresponding to the main circuit switching elements; free-wheel diodes 5u to 5w and 5x to 5z are connected in anti-parallel between the collector and emitter of the MOSFETs 4u to 4w, 4x to 4z; the load 6 (for example a motor) is connected on the output side of the main inverter circuit 3.
A control circuit (to be described) is provided that controls the main inverter circuit 3. This control circuit is positioned as a sub-circuit (auxiliary circuit) in regard to the main inverter circuit 3 referred to above.
Reverse voltage application circuits 7 are connected with the respective free-wheel diodes 5u to 5w, 5x to 5z. These reverse voltage application circuits 7 comprise low-voltage DC voltage sources 8 of lower voltage than the DC voltage source 1; power lines 8a, 8b of the low-voltage DC voltage sources 8 are respectively connected between the collector and emitter of the MOSFETs 4u to 4w, 4x to 4z. 
Each reverse voltage application circuit 7 comprises a base drive circuit 9; the power lines 9a, 9b of the base drive circuit 9 are connected with the power lines 8a, 8b of the low-voltage DC voltage source 8, so that, when drive signals SGu to SGw, SGx to SGz (not shown) are output to the base drive circuit 9 from a switching timing generating circuit, not shown, the base drive circuit 9 is driven by the power from the low-voltage DC voltage source 8, and the MOSFETs 4u to 4w, 4x to 4z are turned ON.
The reverse voltage application circuits 7 comprise MOSFETs 17 corresponding to reverse voltage application switching elements; the MOSFET 17 is interposed on the power line 8a of the low-voltage DC voltage source 8; for the MOSFETs 17, MOSFETs of a lower voltage withstanding ability than the MOSFETs 4u to 4w, 4x to 4z are selected. These MOSFETs 17 are turned ON during reverse recovery of the free-wheel diodes.
The reverse voltage application circuits 7 comprise a diode 13 and capacitor 14; these diodes 13 and capacitors 14 are connected in parallel with the power lines 8a of the low-voltage DC voltage sources 8, so that, when the MOSFETs 4u to 4w, 4x to 4z are turned ON, the capacitors 14 are charged from the low-voltage DC voltage sources 8 through the diodes 13. In this way, the drive power of the base drive circuit 18 is charged on the capacitors 14. A capacitor 15 is connected between the power lines 8a, 8b and a diode 29 is connected in series with the power line 8a. Also, a diode 16 is connected between the power lines 8a, 8b. 
The power lines 18a, 18b of the base drive circuit 18 are connected with both terminals of the capacitor 14, so that, when drive signals SGru to SGrw, SGrx to SGrz (not shown) are output to the base drive circuit 18 from potential discrimination circuits (voltage discrimination circuits), not shown, that output drive signals in response to the potential of the points A, B, C of the main inverter circuit 3, the base drive circuit 18 is driven by the charged power of the capacitor 14, thereby turning the MOSFET 17 ON. In this way, reverse voltage smaller than the DC voltage source 1 is applied from the low-voltage DC voltage source 8 through the MOSFET 17 to the free-wheel diodes 5u to 5w and 5x to 5z. 
However, in the case of such a conventional device, in order to operate the reverse voltage application circuit 7 during reverse recovery of the free-wheel diodes, it is necessary to ascertain the direction of the main circuit current by detecting the potential at the points A, B, C of the main inverter circuit 3, so a voltage detector is required.
Furthermore, during reverse recovery of the free-wheel diodes, the current of the main circuit temporarily flows to the low-voltage DC voltage power source 8 of the reverse voltage application circuit, so there is considerable voltage fluctuation of the auxiliary power source. Specifically, during reverse recovery of the free-wheel diodes, the current that is flowing in the free-wheel diodes is prevented from flowing by the reverse voltage application circuit 7, so the current that was flowing in the free-wheel diodes temporarily flows into the reverse voltage application circuit 7, with the result that a circuit is formed whereby the free-wheel diodes are bypassed through the low-voltage DC voltage power source 8 of the reverse voltage application circuit. As a result, considerable voltage fluctuation of the low-voltage DC voltage power source 8 of the reverse voltage application circuit is produced. As a result, the current capacity of the low-voltage DC voltage power source 8 of the reverse voltage application circuit must be made large.
An object of the present invention is to provide a power conversion device wherein the main circuit current flowing into the auxiliary power source of the reverse voltage application circuit during reverse recovery of the free-wheel diodes is suppressed and reverse recovery of the free-wheel diodes can be appropriately performed.
A further object of the present invention is to provide a power conversion device wherein reverse recovery of the free-wheel diodes can be appropriately performed without providing a detector to detect the direction of the current flowing through the free-wheel diodes and wherein the current of the main circuit flowing to the auxiliary power source of the reverse voltage application circuit during reverse recovery of the free-wheel diodes can be suppressed.